Graduate Thesis Or Dissertation
 

A study of nisin resistance in Listeria monocytogenes Scott A

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  • Eight foodborne pathogenic and spoilage Gram-positive bacteria were evaluated for their spontaneous resistance frequencies to the peptide antimicrobial nisin. In brain heart infusion (BHI) medium, nisin resistance frequencies were in the range of l06 to l08 when exposed to nisin at concentrations 2 to 4 times the minimal inhibitory concentrations. A stable nisin resistant mutant of Listeria monocytogenes Scott A was obtained by increasing stepwise exposure to nisin and subsequently characterized. Phospholipid content, fatty acid composition, phase transition temperature (Tc), and specific growth rates of the resistant mutant and parent were determined. The Tc of the resistant mutant (44.4°C) in comparison of the parent (37.4°C) indicated that significant changes occurred in the lipid composition of the mutant. Gas chromatographic analysis of fatty acids of mutant and parent revealed significant differences (P<0.05) in the proportional ratios of fatty acids; 14:0, 15:0 iso, 15:0 anteiso and 17:0 iso. cells and the concentration of nisin required for reaching a 50% maximum killing rate was five times that of sensitive cells. The bactericidal activity of nisin was more effective at 37°C than 4°C for both resistant and sensitive cells, but there was always higher survival of resistant cell than sensitive cells for both temperatures. Nisin degradation and plasmid involvement were demonstrated to not be the responsible resistance mechanism to nisin in L. monocytogenes. No cross-resistance was observed to therapeutically important antibiotics (rifampicin, chloramphenicol, vancomycin, erythromycin) and to metabolic inhibitors (CCCp, DCCD) by the nisin-resistant mutant.
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